Haven't been to Hanoi, only Ho Chi Mihn (Saigon) and Nah Trang (coastal city northeast of Saigon). Action was plentiful in the Saigon bars, people are friendly, many speak English and things are on the inexpensive side. Nah Trang was a lively & pretty nice beach town with some good bars but not much girly action and things were dirt cheap. My buddy and I did pick up a couple of locals at one of the bars the last night we were there. After a few drinks, rode on the back of their scooters to our hotel, paid the night doorman a couple of bucks to let us bring the girls in and park their scooters in the lobby. My girl stayed all night and she asked for a bit of money in the morning. Gave her my last 500K Dong bill (About $20), was happy as a clam, banged me one more time and then left. If you do go to the coastal areas, I think the best MO is to find a girl in the big city and take her with you though.

Maybe take a look at ISG for info on Hanoi although I do hear it's a bit more conservative up there.